This invention relates to a triac switching circuit used to control the application of AC power to loads and, more particularly, to a system for testing the operability of a triac switching circuit while the circuit performs its switching function.
Industrial control systems need to apply AC power to a load, such as a motor, for a controlled period of time and to reliably turn the application of the power to the load off and on. For example, in a motor-operated valve controller, a motor is energized for a selected period of time to position the valve at a desired set point. Other examples are induced draft and forced draft fans in power plants. Misoperation of such fans can damage the boiler or duct work of the power plant. In typical systems, the turning off and on of the AC power to the load is done by triacs. When a triac fails, it usually fails in the form of a short circuit causing the AC power to remain applied to the load. When the control system calls for the application of power to the load to be terminated under some circumstances, this situation can be hazardous. The triac can also fail in an open position. This condition would manifest itself eventually as failure of the equipment to be powered. It is desirable, however, to have an indication of this kind of failure as soon as the triac fails for safer operation.